After some digging I found https://github.com/flatpak/flatpak/issues/5462 and alas: sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0 makes thumbnail generation work again.
After some digging I found /github. com/flatpak/ flatpak/ issues/ 5462 apparmor_ restrict_ unprivileged_ userns= 0
https:/
and alas:
sudo sysctl -w kernel.
makes thumbnail generation work again.